In the last few years, ultra short pulsed laser systems have become widely available for commercial applications. One application of such ultra short pulsed lasers involves intra-material ablation. For intra-material ablation, the local electrical field strength at the focal point of the laser (usually measured in J/cm2 or equivalent units) must be equal to or greater than the binding energy of the material's valence electrons to their atoms. When the beam's energy density is equal to or greater than the material's energy density threshold, a laser induced optical breakdown (LIOB) of the material occurs at or near the focal point. During LIOB, a microplasma, gas bubbles and shockwaves are generated.
With the above in mind, consideration is given here for the use of an ultra short pulsed laser for photoablating a selected material without affecting the adjacent non-selected material. Specifically, consideration is given for the use of such photoablation where adjacent materials have different ablation thresholds.
In considering details of a photoablation procedure, the geometry and intensity distribution of a laser beam should be understood. In particular, the shape of the laser focus can be assumed to be hyperbolic, and the intensity distribution can be assumed to be Gaussian. With these assumptions, the laser focus has a minimum radius (the beam waist) and has a length (or focal depth). The boundary of the laser beam is usually defined by the position where the intensity of the beam has decreased to 1/e2 of the intensity in the center of the beam.
With a known ablation threshold for LIOB, it might be expected that the location where LIOB occurs within the laser focus can be simply determined by the position where the necessary energy density is reached. Therefore, the position of LIOB within a material could be changed not only by moving the position of the laser focus, but also by altering the beam energy. In other words, the intensity needed for LIOB scales with the size of the laser focus. However, this relationship is based on the assumption that the whole energy of the laser pulse is instantaneously deposited within the material. In reality, the intensity (and, thus, the energy) of the laser beam also has a distribution along the z-axis that is determined by the pulse length and shape of the laser pulse and can be described by a sech2-curve. This leads to a spatial distribution of the intensity along the z-axis. As a consequence, LIOB occurs only when the intensity of the laser pulse reaches the threshold intensity needed for LIOB.
If the threshold for LIOB changes along the z-axis (e.g., at the interface between two materials), then the position of LIOB changes significantly. For instance, a laser may induce LIOB at its focal point in one material but have no effect when another material is used. Therefore, appropriate settings for beam geometry and beam energy can be used to control the position of LIOB at the interface of two materials having different ablation thresholds. If the ablation thresholds for the materials are known, or can be identified, the energy level can be calculated to provide LIOB in only one of the materials. Furthermore, by identifying the position of the interface before activating the laser, the focal point may be initially positioned in the targeted material with the appropriate energy level to avoid any unintentional ablation. After the laser beam has been activated, detecting means may be used to detect whether LIOB has occurred and, if so, in which material. Such detection may be through analysis of the size of the bubble resulting from LIOB or through spectral analysis of the plasma resulting from LIOB.
While intra-material photoablation may be performed on various materials, its use on corneal tissue or biological tissue is of particular interest. With regard to corneal tissue, it is noted that several surgical procedures exist for modifying its structure. To understand these procedures, the operation and anatomy of the cornea should be understood.
Along with the lens, the cornea refracts incoming light and focuses the light on or near the retina. The curvature of the cornea determines where the incoming light will be focused. If the curvature of the cornea is too steep or too flat, it may be modified by photoablating certain corneal tissue. Anatomically, the structurally distinct corneal tissues include, in order from the anterior to the posterior of the eye, the epithelium, Bowman's membrane, the stroma, Descemet's membrane, and the endothelium. Of these, the stroma is the most extensive, being generally around four hundred microns thick. Consequently, the stroma provides the most opportunity for correction via photoablation. Additionally, the healing response of the stroma is typically quicker than the other corneal layers.
In the past, techniques such as laser-assisted in situ keratomileusis (LASIK) and laser epithelial keratomileusis (LASEK) have been used to reshape stromal tissue. In these procedures, stromal tissue is ablated after being exposed by temporarily removing the overlying tissues. Other ophthalmic procedures rely on subsurface photoablation, i.e., the photoablation of stromal tissue without first exposing the tissue through the removal of overlying tissue. Because the subsurface photoablative procedures provide clear benefits over the prior art's reliance on the removal of overlying tissues, further efforts have been made to utilize intracorneal photoablative techniques. Specifically, these efforts have involved the use of ultra short pulsed laser systems in intracorneal photoablation procedures. As a result of these efforts, an intracorneal technique has been discovered that allows precise separation of corneal tissues along their interface using an ultra short (femto-second) pulsed laser.
In light of the above, it is an object of the present invention to provide an efficient surgical method for creating a discontinuity at the interface between two distinct materials. Another object of the present invention is to provide a method and device for separating two distinct materials having different ablation energy thresholds. It is yet another object of the present invention to provide a surgical method and device for creating a corneal flap that allows for the accurate positioning of the corneal flap at a predetermined location on the cornea. Still another object of the present invention is to provide a method for intra-material photoablation along an interface that is easy to perform and is comparatively cost effective.